Plasma arc torches often are operated underwater to reduce the noise associated with plasma cutting and minimize the adverse environmental impact of the cutting process. The water traps the plasma generated emissions and particulates that otherwise would be discharged into the air. Additionally, underwater cutting reduces the amount of harmful glare and ultraviolet radiation exposed to workers.
However, underwater plasma arc cutting requires increased power requirements for torch operation at reduced rates of feed. Most importantly, the torches must be designed to protect the discharged plasma from water which tends to flow into the cutting zone. Not only is the cut quality reduced when water enters the cutting zone, but gaseous by-products such as hydrogen are generated, possibly resulting in small hydrogen detonations under the workpiece.
In one proposed plasma arc torch disclosed in U.S. Pat. No. 4,816,637, the torch includes an attachment which provides a flow of gas outward from the nozzle in enveloping relation to the plasma discharged from the nozzle. The gas is discharged downward through axial holes and through a substantially annular opening in inward circumferential relation to the plasma.
This flow forms a "gas bubble" or air curtain around the arc and minimizes the amount of water which can flow within the cutting zone. A flow of water having a velocity greater than 8.7 feet/second is then generated in spaced relation to the discharged gas to aid in controlling the shape of the flow of gas, and to aid in reducing the noise generated during cutting when the torch is used above water.
Although the water flow into the cutting zone is minimized with this proposed torch, the air curtain is not evenly formed throughout its circumference because the gas is discharged through individual gas discharge holes, producing multiple flows of gas. As a result, water may flow through the weaker areas of the bubble created by the lack of uniform gas flow within the annular air channel. Additionally, because the high velocity gas is discharged towards the plasma in multiple flows of gas, the gas impinges on the plasma without first expanding outward, creating a smaller, less desirable diameter air curtain, thus increasing the chance that water may flow into the cutting zone.
In U.S. Pat. No. 4,291,217, the plasma arc is enveloped by a compressed air sheath outside the nozzle. The compressed air flows through gas discharge openings evenly distributed around the nozzle and forms an uneven air curtain that provides some measure of protection against water entering the cutting zone. However, this proposed construction also may have some water passing into the cutting zone because the formed air curtain is uneven and additionally, smaller in diameter than desired.